1. Field of the Invention
The present invention concerns a method for implementation of a magnetic resonance examination of a patient with an imaging medical magnetic resonance apparatus with a movable patient bed, wherein an examination volume of the patient to be examined is larger than the acquisition volume of the magnetic resonance apparatus.
2. Description of the Prior Art
A goal of magnetic resonance tomography and magnetic resonance spectroscopy is to implement patient examinations that are ever shorter and are conducted with stronger gradient magnetic fields that are produced with shorter gradient field systems (coils). A problem occurs in the examination of larger body regions, such as the entire patient or non-contiguous regions thereof, because the acquisition volume in the MR scanner becomes smaller as the gradient field system becomes shorter. The magnetic resonance apparatuses (MR scanners) that are advantageous with regard to interventional diagnoses and with regard to the increased patient comfort thus place new requirements on the process of an examination with such a magnetic resonance apparatus. The problem of conducting an examination of an examination region that cannot be acquired with a local acquisition is solved by moving the patient through the acquisition volume by means of a movable patient bed. The movement can ensue in steps or continuously. In the former case conventional, local acquisition techniques for an a non-moving subject can be used.
A method for data acquisition of a large acquisition volume using a continuously moving patient bed is known from PCT Application WO 02/082996. A seamless image of the acquisition region is thereby acquired.
Imaging with a continuous movement of the patient bed (acquisition of a moving subject) is, for example, known from “Helical MR: Continuously Moving Table Axial Imaging With Radial Acquisitions”, A. Shankaranarayanan et al., Magnetic Resonance in Medicine, 50; 1053 to 1060, 2003.
The conventional magnetic resonance technique deals with local examinations of individual body regions in the isocenter of the basic field magnets, i.e. in the region of the highest homogeneity of the basic magnetic field. If various body regions should be measured, the patient is either shifted with the aid of the patient bed or is repositioned. After each shifting, multiple position-dependent parameters are re-detected in a localizer measurement (scan) in order to derive a measurement protocol therefrom. Measurement planning over the entire patient region thus is not possible. If various regions are measured, the dwell time of the patient in the magnetic resonance apparatus is prolonged due to the respective localizer measurements being re-implemented.
In addition to fundamental adjustment parameters of the apparatus, to an increasing degree calibration measurements are necessary for implementation of magnetic resonance examinations. These enable the determination of sensitivity profiles of coils that can be used for image brightness compensation, for parallel imaging or for coil position determination. Furthermore, distributions of the basic magnetic field, the radio frequency field and/or gradient magnetic fields can be determined for correction of, for example, gradient echo images.
In connection with magnetic resonance measurements with a continuously moving patient bed, in the aforementioned article “Helical MR: Continuously Moving Table Axial Imaging With Radial Acquisitions” it is noted that more precise technical measurement parameters could be acquired with localizer measurements for various positions of the patient bed in order to improve the image quality. For this purpose, it is proposed to acquire (in a preceding measurement) and store shim values, the center frequency of the MR excitation, and reception and transmission amplifications for various body parts. These discrete values can then be interpolated and adapted as necessary as the different body parts move through the acquisition region.
A method for determination of measurement values with adherence to SAR limit values is known from German OS 101 50 138, in which the bed and therewith the adjusted field of view is moved relative to the transmission antenna. The position of the patient is determined by pattern recognition from the associated pre-measurements and is used for calculation of SAR values. A disadvantage of this method is that the acquired valued is valid only for a specific table position. Indeed, this value can quite significantly change given a small table shift.